With the spread of personal computers, the increased use of floppy disk apparatuses, hard disk apparatuses and optical disk apparatuses as external memories is remarkable. In particular, the demand for hard disk apparatus having short access time and large capacity increases.
The hard disk apparatus comprises a spindle motor, a disk, a head actuator, a magnetic head, a disk enclosure, a breathing filter, a position control circuit and a recording and reproducing circuit. The head actuator is an important device for positioning the magnetic head with good precision and high-speed.
Hitherto a voice coil motor and a step motor have been mainly used as the actuator for the hard disk.
The using of the voice coil motor has an advantage in that it can easily obtain a precise position control because it has no cogging, but it has a disadvantage in that the size and the power consumption necessary to obtain the desired driving force is large.
On the other hand, using the step motor has an advantage in that the driving circuit is simple, because it is usable with an open loop motor driving system and the size efficiency of the driving force is better than that of the voice coil motor. However, it has the disadvantage of being inadequate for precise position control. Further, the step motor can rotate a rotor in such a way as to correspond to a period of external pulses, sequentially changing a magnetic stable status by changing the conducting mode of the coils, by means of the external pulses. However, the step motor has the disadvantage of an asynchronous phenomenon.
The asynchronous phenomenon is that the rotor can not follow the speed of the external block because the conducting mode shifts to the next mode if the switching of the coil conducting mode is faster than a predetermined speed in case the rotor does not reach the magnetic stable point, or in case the rotor is in a transient state, to a vibrationally stable status.
In order to resolve the above-mentioned problem, closed loop driving of the step motor is being widely used. This is a method such that, by providing an optical or a magnetic position detector at the outside of the step motor, electrification of the coil is controlled by the output of the detector, thereby enabling rotation up to high speed without asynchronism.
However, in the above-mentioned method using the external position detector, the size of the actuator is large because the position detector is provided independently of the motor part, and the detected signals are liable to fluctuate on account of the irregularity of positions of magnetizations of the magnetic drum of the position detector. Further, the accurate positioning of the position detector on the rotor is difficult.
As mentioned above, the voice coil motor used in the office automation apparatus, particularly in the hard disk apparatus, has disadvantages such as large size and large power consumption. The step motor widely used in the office automation apparatus also has disadvantages, such as difficulty of obtaining fine positioning and inducing asynchronous phenomena. Further, when the position detector is attached to the step motor, the size is large and it is troublesome to attach the position detector.